1. Field of the Invention
The present invention relates to an exchangeable lense to be mounted on a camera through a bayonet mount mechanism, and more particularly to an exchangeable lense provided with a group of terminals through which electrical signals are transmitted and electric power is supplied from the camera body to the exchangeable lens.
2. Description of the Prior Art
A known exchangeable lens of the above mentioned type such as disclosed in a Japanese laid-open patent application with the laid-open No. 58-83824, is provided with a plurality of electric terminals arranged circularly along the circumference of the lens barrel about the optical axis of the lens. These terminals are arranged to enable desired exchange of electric signals between the camera body and the exchangeable lens when the exchangeable lens has been turned to a completely attached mounted position relative to the camera body with the terminals on the lens having been brought into registration with corresponding terminals on the camera body. To this end, the terminals on the exchangeable lens and the terminals on the camera body are respectively arranged circularly about the optical axis mutually at corresponding positions such that all the respectively corresponding terminals come into contact with each other simultaneously just before the completion of the coupling of the exchangeable lens to the camera body.
In cases where an electric power supply system is arranged such that the electric power is supplied from the camera body through the above stated terminals to the exchangeable lens circuit, it is desirable that the lens circuit consumes less electrical power. To this end, the lens circuit may comprise a CMOS (Complementary Metal Oxide Semiconductor). However, CMOS circuits have a defect in that they are susceptible to an undesirable circuit phenomenon known as "latch up" phenomenon. The latch up phenomenon occurs when the combination of the source, drain, input protection resistor, P-well, substrate, and the like constituting the CMOS acts as if it is a thyristor due to the triggering of a surge, or the like. A detailed analysis of the latch up phenomenon will be given later on. When the latch up phenomenon occurs, the power supply terminal and the ground terminal of the CMOS are short-circuited with each other, and an excessive flow of electric current is created. Since this condition continues until the power supply is interrupted, the power source of the camera body is rapidly exhausted. Further, it is likely to occur that the CMOS itself is broken by the excessive electrical current and the communication of the electric signal between the camera body and the exchangeable lens becomes impossible.
Generally, a protection resistor or a protection diode is provided for the input/output terminal to prevent the latch up phenomenon, however this countermeasure by itself is not completely effective in preventing the phenomenon.